1. Field of the Invention
The present invention relates to a micro inertia sensor and a method thereof, the micro inertia sensor such as a micro gyro comprising a device wafer used as a lower structure, a cap wafer used as an upper structure, and a bonding and wiring structure thereof. The present invention provides a micro sensor being in a new structure as miniaturized in comparison to the existing structure and enabling to sense the up-and-down movement.
2. Description of the Related Art
In general, in the related art of manufacturing a micro sensor such as a micro gyro, the way of accomplishing a wafer-level packaging is used by anodically bonding a silicon micro structure formed on the silicon or glass wafer to the glass wafer.
However, in such a related method, there is one problem that a bending occurs on the bonded surface due to a different coefficient of thermal expansion between the silicon. There is also the other problem such as degassing phenomenon that oxygen generates from the glass when anodic bonding is applied.
Therefore, in order to solve the above-mentioned problems, eutectic bonding between silicon (Si) and gold (Au) is used instead of anodic bonding between glass and silicon. In case of applying the eutectic bonding, there is no stress between the bonded surfaces, thereby improving yield; and there occurs no degassing, thereby enabling to maintain a high vacuum. As a consequence, it has the merit of improving the features of the inertia sensor such as gyro.
In the related art of carrying out the eutectic bonding, a device wafer is manufactured by forming an insulation layer on a first silicon; forming a second silicon on the insulation layer; and patterning, etching and forming a structure body to sense the movement at a border, fixed points and sides.
To prevent any pollution in the process on the device wafer and to keep a sealed state, a cap wafer is manufacture by forming a first insulation layer on a third silicon; forming an electric conduction wiring on the first insulation layer; forming a second insulation layer on the electric conduction wiring; and evaporating Cr/Au layer. A micro inertia sensor is manufactured by bonding the device wafer to the cap wafer by means of the eutectic bonding.
An embodiment according to the related art will be described in detail, on the basis of the attached drawings.
FIGS. 1axe2x80x941f are outlined schematic diagrams to illustrate a hermetic packaging process of a micro sensor by forming a device wafer and a cap wafer according to the related art. FIG. 2 is a front cross-sectional structure view illustrating a bonding state of the device wafer to the cap wafer in the hermetic package, wherein a groove portion is formed in the sealing bonded portion of the micro sensor according to the related art.
FIG. 1a illustrates a sensor""s surroundings of a silicon device wafer 140 including a silicon 110 being about 500 xcexcm in thickness, an insulator 120 of an oxidation film being interposed on the upper portion of the silicon 110, a micro sensor 130 being provided on the upper portion, and an internal fixed point 150 and an external fixed point 160 being formed at regular intervals. Here, a sealing bonding portion 170 having a low melting point, which is sealed by the micro sensor when bonding to a cap wafer 210, is formed between the internal fixed point 150 and the external fixed points 160. A groove 300 forms by etching and removing the lower side of the sealing bonding portion 170.
The device wafer 140 is manufactured and accomplished by removing the insulator 120 excluding the internal fixed point 150, the external fixed point 160 and the sealing bonding portion 170 of a low melting point by means of the etching process as illustrated in FIG. 1b. 
A cap wafer 210 of silicon being in regular thickness, which is to bond to the device wafer 140 is provide and insulator 220 and an electric conduction film 230. The insulator 220 is evaporated on the upper portion of the cap wafer 210. The electric conduction film pattern 230 to be bonded to the internal fixed point 150 and the external fixed point 160 of the device wafer 140 is formed on the upper portion of the insulator 220. FIG. 1c shows the conductive film pattern.
A second insulator 220xe2x80x2 is evaporated on the upper portion of the electric conduction film pattern 230 to protect the conductive film; and Cr/Au is evaporated on the upper portion of the electric conduction film pattern 230 for patterning (as shown in FIG. 1d).
In FIG. 1f, a wire bonding is provided by etching the outside of the portion of the cap wafer 210 to be connected with the external fixed point 160 of the device wafer 140, wherein the etching is performed by photo-resist not to effect on Cr/Au.
After the device wafer 140 and the cap wafer 210 are manufactured as above, as shown in FIG. 2, heat of 400xc2x0 C. and pressure are applied to the device wafer 140 and the cap wafer 210. Then, due to the low melting point bonding, Au melts and seals the micro sensor 130 of the device wafer 140. Having conductivity, Au performs two roles of bonding and electric conduction at a time.
Auxe2x80x94Si alloy as melted by heat and pressure by the low melting point bonding is gathered on the corner of the groove 300 formed on the sealing bonding portion 170 when the device wafer 140 is bonded to the cap wafer 210, thereby improving the sealing of the micro inertia sensor 130 positioned on the device wafer 140.
However, the related art as described above has the problems that it is difficult to miniaturize the micro sensor and it is complicate to manufacture and fix it. That is, when power is supplied to the external fixed point 160 as the silicon (Si) layer, the power is connected to the internal fixed point 150 through the wiring between the insulation layers 220 and 220xe2x80x2. Hereby, the change of the side movement is sensed according to the variation of the capacitance. In this regard, the external fixed point 160 must necessarily exist for the power supply from the outside, and the space for the external fixed point 160 must be necessary secured.
There is the trial to form electrodes by removing the external fixed point and forming a via hole on the cap wafer. However, such a trial has a risk to touch the wiring which passes between the insulation layers since the via hole should be most deeply formed to prevent the effects of occurrence such as undercut. In this connection, as the via hole cannot be properly made on the place as desired, there still exits the problem that it is difficult to reduce the size.
It is, therefore, an object of the present invention to provide a micro inertia sensor forming a device wafer on the lower portion and a cap wafer (SOG: silicon on glass) on the upper portion as the MEMS structure body; and a via hole in the direction from the upper surface of the cap wafer to the device wafer, thereby providing a micro inertia sensor miniaturized and manufactured in a simplified manner.
To achieve the above object, there is provided a micro inertia sensor includes a lower glass substrate; a lower silicon including a first border, a first fixed point and a side movement sensing structure; an upper silicon including a second border, a second fixed point being connected to a via hole, in which a metal wiring is formed, on an upper side, and an sensing electrode, which correspond to the first border, the first fixed point and the side movement sensing structure; a bonded layer by a eutectic bonding between the upper silicon and the lower silicon; and a upper glass substrate, being positioned on an upper portion of the upper silicon, for providing the via hole on which an electric conduction wiring is formed.
The side movement sensing structure comprises a structure being movable in a horizontal direction and a sensing electrode senses a variation of a capacity as the structure horizontally moves, while the sensing electrode senses a capacity in a vertical direction.
Therefore, the inertia sensor can be made of enabling to horizontally and vertically sense on the space of axes x, y and z.
The bonded layer by eutectic bonding in the preferred embodiment of the present invention is characterized by forming by Au-Si eutectic bonding.
That is, the micro inertia sensor comprises the device wafer and the cap wafer with the SOG (silicon on glass) wafer; the via hole formed from the cap wafer; and the sensing electrode senses the upper portion formed on the cap wafer, thereby solving the difficulty in the existing electrode wiring and enabling to miniaturize and to sense the vertical capacity. Specifically, the related art has one problem that the electrode wiring is touched when the via hole is formed and the other problem that a point of contact is not generated due to the effects of undercut. However, those problems are solved in the present invention. In addition, when the via hole is formed from the glass substrate on the SOG wafer to the silicon, the via hole is formed to the silicon layer, and therefore the operating electric current sufficiently flows, extending to the silicon layer.
In addition, the device waver and the cap wafer are made of the SOI (silicon on insulator) wafer, LPCVD polysilicon or Epi polysilicon.
In another aspect of the present invention, there is provided a method of manufacturing the micro inertia sensor comprising the steps of:
forming a device wafer by forming a lower silicon on a lower glass substrate; etching the lower silicon for forming a side movement sensing structure including a structure being movable in a horizontal direction on the lower silicon and an sensing electrode for sensing a variation of a capacity as the structure horizontally moves, a first fixed point, and a first border for bonding; etching the lower glass substrate as a sacrificial layer; and separately evaporating Au for bonding on the lower silicon layer;
forming a cap wafer by forming an upper silicon on an upper glass substrate; forming a gap in the upper silicon; forming an second fixed point, an second border and a second sensing electrode, which correspond to the first fixed point, the first border and the structure movable in a horizontal direction in the device wafer process; and forming the via hole from an upper glass substrate to the second fixed point; and
bonding the device wafer and the cap wafer by a eutectic bonding; evaporating an electric conduction layer on the via hole to form an electric conduction wiring.
According to the present invention, Au for eutectic bonding can be formed by a pre-pattern before the structure is formed.
According to the present invention, the metal wiring formation can be pre-progressed before bonding but after forming the via hole.